As the use of the Internet spreads, the amount of data communicated is rapidly increasing, making the optical fiber communication very important. In the optical fiber communication, electric signals are converted into optical signals, and the optical signals are transmitted through optical fibers. The optical fiber communication is characterized in that the signals are transmitted in the broad band, with a small loss, and are not affected by noise.
Known as systems for converting electric signals into optical signals are the direct modulation system using a semiconductor laser and the external modulation system using optical modulators. The direct modulation system need not use the optical modulators and its running cost is low, but cannot achieve high-speed modulation. This is why the external modulation system is used in high-speed and long-distance data communication.
As the optical modulator, a Mach-Zehnder optical modulator in which an optical waveguide is formed by Ti (titanium) diffusion in the vicinity of the surface of a single-crystal lithium niobate substrate has been practically used (see, e.g., Patent Document 1). Although high-speed optical modulators having a modulation speed of 40 Gb/s or more are commercially available, they have a major drawback that the entire length thereof is as long as about 10 cm. The Mach-Zehnder optical modulator is an optical modulator that uses an optical waveguide (Mach-Zehnder optical waveguide) having a Mach-Zehnder interferometer structure. The Mach-Zehnder interferometer is a device that separates light emitted from one light source into two beams, makes the two beams pass through different paths, and then recombines the two beams to cause interference, and the Mach-Zehnder optical modulator applying the Mach-Zehnder interferometer is used for generating various modulated lights.
On the other hand, Patent Documents 2 to 4 disclose a Mach-Zehnder optical modulator using a c-axis oriented lithium niobate film. The optical modulator using the lithium niobate film achieves significant reduction in size and driving voltage as compared with the optical modulator using the lithium niobate single-crystal substrate.
As described in Patent Document 1, conventional Mach-Zehnder optical modulators have a problem of deterioration in signal waveform due to a change in wavelength of a modulated light called wavelength chirp. Electric fields to be applied to respective parallel waveguides differ from each other in strength due to a difference in arrangement of a signal electrode relative to the parallel waveguides, with the result that a variation amount (Δns) of the refractive index of one waveguide that is close to the signal electrode becomes larger than a variation amount (ΔnG) of the refractive index of the other waveguide that is far from the signal electrode. This makes the phase changes of lights propagating in the respective parallel waveguides differ in absolute value, so that the wavelength chirp occurs to degrade a signal waveform after transmission when a signal is changed from “0” to “1” or from “1” to “0”.
The cross-sectional structure of a conventional optical modulator 600 described in Patent Document 2 is illustrated in FIG. 9A. A pair of optical waveguides 22a and 22b of a lithium niobate film are formed on a sapphire substrate 21, and a signal electrode 24a and a ground electrode 24b are disposed above the optical waveguides 22a and 22b, respectively, through a buffer layer 23. The optical modulator 600 is of a so-called single drive type having one signal electrode 24a, and the signal electrode 24a and ground electrode 24b have a symmetrical structure, so that electric fields to be applied to the optical waveguides 22a and 22b are the same in magnitude and opposite in polarity, preventing the wavelength chirp of a modulated light from occurring. However, it does not operate at high frequencies because the area of the ground electrode 24b is small.
The cross-sectional structure of a conventional optical modulator 700 described in Patent Document 3 is illustrated in FIG. 9B. Two signal electrodes 24a1 and 24a2 are disposed above a pair of optical waveguides 22a and 22b of a lithium niobate film, respectively, through a buffer layer 23, and three ground electrodes 24c 24d, and 24e are disposed so as to be separated from the signal electrodes 24a1 and 24a2. When voltages same in magnitude and opposite in polarity are applied to the two signal electrodes 24a1 and 24a2, respectively, electric fields to be applied to the optical waveguides 22a and 22b become the same in magnitude and opposite in polarity, preventing the wavelength chirp of a modulated light from occurring. Further, the amount of the chirp can be adjusted by adjusting voltage to be applied to the pair of optical waveguides 22a and 22b. Furthermore, since the areas of the left and right ground electrodes 24c and 24d are sufficiently ensured, it has a structure operable at high frequencies. However, the optical modulator 700 is of a dual drive type having two signal electrodes 24a and 24b, which complicates an electrode structure. Further, it is necessary to provide two input connectors for high-frequency electric signals and to apply voltages to both the signal electrodes while controlling the phase of the electric signal in which data has been inversed, and this complicates the circuit configuration of a drive system.